In the manufacturing for semiconductor products, such as semiconductor integrated circuits, liquid crystal panels, solar battery panels, and magnetic disks, plasma which is generated by high frequency discharge in rare gas atmosphere is used in various steps. In the processes, argon has been used conventionally.
However, krypton and xenon have attracted attention, for the purpose of performing an advanced manufacturing process.
However, krypton and xenon are extremely expensive gases because of the abundance ratio in the atmosphere and the complexity of the separation process. Discharging into the atmosphere after using as an atmospheric gas is problematic in that the cost increases remarkably. For the process using these expensive gases to be economical, it is extremely important to recover the rare gas used with a recovery percentage of 99% or greater and use cyclically. In order to reuse the recovered rare gas, a concentration of 99% or greater is required.
The exhaust gas from the manufacturing equipment for semiconductor products and displays, etc. mainly contains an atmospheric gas and a purge gas introduced into the manufacturing equipment during the evacuation. The exhaust gas further contains gas introduced depending on the manufacturing method for semiconductor products. For example, when an oxide film is produced, the exhaust gas further contains oxygen. When a nitride film is produced, the exhaust gas further contains nitrogen, and hydrogen or ammonia. When a plasma CVD is performed, it further contains metal hydride gas. When a reactive ion etching is performed, it further contains halogenated carbon hydride gas, helium, and nitrogen. In addition, when a plasma process is performed, it further contains moisture, carbon dioxide, hydrogen, NOx, and carbon hydride, etc. as by-products.
In general, a pressure swing adsorption separation (PSA) method is known as a method for recovering a target component from mixed gas. As a typical device using a pressure swing adsorption separation method, there are an oxygen PSA device and a nitrogen PSA device. In these typical PSA devices, a strong adsorbate is removed, and a weak adsorbate is recovered as a product. In an oxygen PSA device, nitrogen which is a strong adsorbate is removed and oxygen which is a weak adsorbate is recovered using zeolite as an adsorbent. In a nitrogen PSA device, oxygen which is a strong adsorbate is removed and nitrogen which is a weak adsorbate is recovered using an adsorbent such as CMS (carbon molecular sieve).
A target component (weak adsorbate) can be concentrated in a high concentration in these typical PSA devices. However, the weak adsorbate which remains in spaces made by the adsorbent particles or by the adsorbent as a co-adsorbed ingredient is exhausted with the strong adsorbate in a desorption step. Therefore, a target component (weak adsorbate) cannot be recovered with a high recovery percentage. In order to recover the target component in a high concentration with a high recovery percentage, it is necessary not only to condense the target component, but also to decrease the amount of the target component contained in the exhaust gas as much as possible. In other words, when the mix gas contains two components, a gas separating method which can recover each component in a high concentration with a high recovery is required.
The PSA method using a counter current purge gas is disclosed as a method for recovering plural components from a mixture gas in U.S. Pat. No. 4,599,094 of Werner et al.
The PSA method of Werner et al. recovers a strong adsorbate and a weak adsorbate in a material gas as a product at the same time. For example, when the material gas is air, nitrogen which is a strong adsorbate and oxygen which is a weak adsorbate are both recovered as a product. Specifically, air is supplied with high pressure to the bottom of an adsorption column, and nitrogen which is a strong adsorbate is adsorbed. Oxygen which is a weak adsorbate is led out from the upper portion of the adsorption column. Supplying of air is stopped before an adsorption band of air reaches the top portion of the adsorption column. Then, high concentrated nitrogen gas is supplied from the bottom portion of the adsorption column as a counter current purge gas. The counter current purge gas is supplied until an adsorption band of the high concentrated nitrogen gas reaches above or below the adsorption band of air. In this period, leading out of oxygen from the top of the adsorption column is continued. The adsorption column is saturated with nitrogen which is a strong adsorbate by the counter current purge gas. A part of the oxygen led out from the top portion of the adsorption column is recovered as a product, and the remaining oxygen is used as a counter current purge gas. The nitrogen adsorbed by the adsorbent is stored in a nitrogen storage tank by contacting with oxygen as counter current purge gas under reduced pressure conditions. A part of the nitrogen recovered in the nitrogen storage tank is used as a product, and the remaining nitrogen is used as a counter current purge gas. U.S. Pat. No. 4,599,094 discloses that a nitrogen product having a concentration of 99.8% or greater can be recovered, and an oxygen product having a concentration in a range from 90 to 93.6% can also be recovered.
U.S. Pat. No. 4,810,265 of Lagree et al. discloses a PSA method which is a revised version of the PSA disclosed in U.S. Pat. No. 4,599,094 of Werner et al. U.S. Pat. No. 6,527,830 of Neu et al. discloses a PSA method in which an equalization method and operation conditions are improved.
These methods can improve the nitrogen concentration and the nitrogen recovery percentage which is a strong adsorbate. However, it is impossible to recover oxygen which is a weak adsorbate in the same high concentration as that of nitrogen.
As a method for recovering plural components containing both a strong adsorbate and a weak adsorbate in a high concentration from a mixture gas, a Duplex PSA method is disclosed in U.S. Pat. No. 5,085,674 of Leavitt and U.S. Pat. No. 6,500,235 of Zhong et al.
The Duplex PSA method is characterized in that a material gas is supplied into the middle portion of an adsorption column, a typical PSA method (PSA method for concentrating a weak adsorbate) is performed at the top portion of the adsorption column, and an inverse PSA method (PSA method for concentrating a strong adsorbate) disclosed in U.S. Pat. No. 4,359,328 of Wilson is performed at the bottom portion of the adsorption column.
In the Duplex PSA method, a weak adsorbate which is obtained from the top portion of the adsorption column under high pressure conditions is depressurized and introduced into another adsorption column under low pressure conditions. A strong adsorbate which is obtained from the bottom portion of the adsorption column under low pressure conditions is pressurized, and introduced into the adsorption column under high pressure. That is, gas is cycled between the adsorption column under high pressure and the adsorption column under low pressure. Thereby, the weak adsorbate is concentrated in the upper portion of the adsorption column, and the strong adsorbate is concentrated in the bottom portion of the adsorption column.
The material gas is introduced into the middle portion of the adsorption column. A part of the reflux gas in the upper portion of the adsorption column is recovered as a product of a weak adsorbate. A part of the reflux gas in the bottom portion of the adsorption column is recovered as a product of a strong adsorbate. The Duplex PSA method can recover both a strong adsorbate and a weak adsorbate in a high concentration with a high recovery percentage.
However, when the material gas contains hydrogen or helium, the Duplex PSA method has a problem in that the product of a strong adsorbate contains hydrogen or helium because hydrogen and helium are the weakest adsorbates which are rarely adsorbed by an adsorbent.
In addition to these PSA methods, a PSA method combining an equilibrium separation type PSA method and a speed separation type PSA method is disclosed in Japanese Patent Application, First Publication No. 2002-126435 of Kawai et al. as a method for recovery of a target component with a high concentration and a high recovery percentage.
The PSA method can recover two components as a product by combining the two typical PSA methods (a weak adsorbate is concentrated). For example, when a mixture gas containing krypton and nitrogen is used as a material gas, nitrogen which is a weak adsorbate is recovered by the equilibrium separation type PSA method in which krypton is a strong adsorbate and nitrogen is a weak adsorbate. In addition, krypton, which is a weak adsorbate, is recovered by the speed separation type PSA method in which krypton is a weak adsorbate and nitrogen is a strong adsorbate.
As explained above, nitrogen and krypton can be recovered in a high concentration simultaneously by using adsorbents having different properties and crossing the strong adsorbate and the weak adsorbate. All of the exhaust gas from each PSA device is recovered in a buffer tank, mixed with the material gas, and supplied into each PSA device again. It is disclosed that the method of Kawai et al. can recover krypton in a concentration in a range from 99.9 to 99.99% and nitrogen in a concentration in a range from 97 to 99.9% from a mixture gas containing krypton and nitrogen.
However, when the material gas contains hydrogen or helium, which is a weak adsorbate, it is impossible to prevent hydrogen or helium from being contaminated in krypton, because krypton is recovered by the PSA method for recovering a weak adsorbate.
[Patent document 1]U.S. Pat. Publication No. 4,599,094[Patent document 2]U.S. Pat. Publication No. 4,810,265[Patent document 3]U.S. Pat. Publication No. 6,527,830[Patent document 4]U.S. Pat. Publication No. 5,085,674[Patent document 5]U.S. Pat. Publication No. 6,500,235[Patent document 6]U.S. Pat. Publication No. 4,359,328[Patent document 7]Japanese Patent Application, FirstPublication No. 2002-126435